The process used in the drilling of most oil and gas wells involves the use of a drilling fluid commonly referred to as drilling "mud" in the industry. The mud is injected under pressure through the drill string during drilling and returns to the surface through the drill string-borehole annulus. The mud performs multiple functions which include cooling of the drill bit, lubrication of the drill bit, providing a means of returning the drill cuttings to the surface of the earth and providing hydrostatic pressure to prevent the "blowout" of high pressure geologic zones when such zones are penetrated by the drill bit. Drilling mud comprises a liquid phase and a suspended solid phase. The liquid phase can be either fresh or saline water or even an oil base. The solid phase, which is suspended within the liquid phase, can comprise a multitude of materials blended to meet the particular needs at hand. As an example, barite (barium sulfate), with a specific gravity over 4.0, is often used as a weighting constituent to increase the bulk density of the mud when high pressure formations are being penetrated. Other additives are used to control drilling fluid circulation loss when certain types of high porosity, low pressure formations are penetrated. Once returned to the surface, the drilling fluid contains cuttings from the drill bit. Although most large cuttings are removed at the surface prior to recirculating the mud, smaller sized particles remain suspended within the drilling mud. Upon completion of the drilling operation, the drilling mud can sometimes be reconditioned and used again. Eventually, however, the mud can no longer be reprocessed and becomes classified as a waste product of the drilling operation.
Once the well has been successfully drilled and cased, hydrocarbons are extracted or produced from one or more formations penetrated by the borehole. Although hydrocarbons are the primary production fluids of interest, other non hazardous oilfield waste (NOW) is usually generated in the production of hydrocarbons. A water component is usually produced along with the hydrocarbon component, and in most areas of the world, the produced waters are saline. Although there are some secondary uses for produced waters, these waters are in general considered a waste product of the production operation. Solid wastes including sand, paraffin, sludges and other solid materials are also generated during the production operations. Large quantities of these solid wastes have been accumulated for decades in production pits. Environmental regulations have led to the need for disposal solutions for the materials contained in production pits undergoing remediation to acceptable environmental levels.
The isotopes uranium-238 and thorium-232, and the radioactive isotopes associated with the decay series of these isotopes, occur in nature in earth formations. In situ, the activities associated with these decay chains are relatively low and do not present a radiation hazard during the drilling operation. During well production, however, these naturally occurring radioactive materials (NORM) are dissolved in the produced waters and are transported to the surface. Over an extended period of time, the NORM becomes concentrated in precipitated scale associated with tubulars and surface equipment such as heater treaters, wellheads, separators and salt water tanks. Although the parent isotopes uranium-238 and thorium-232 are not generally present, the decay products or "daughter" products radium-226, radium-228, radon-222 and lead-210 can be found in oilfield waste. Radium-226, which coprecipitates with carbonates and sulfates of calcium, barium and strontium, is by far the greatest source of radioactive waste resulting from production activities. Once atoms of radium have replaced a sufficient number of atoms of the elements normally found in NOW waste to exceed a specified regulatory level, the waste is classified as NORM. Stated another way, there is no difference between NOW and NORM waste other than the level of radioactivity, which usually results from the radium content of NORM waste.
In summary, the drilling and production of oil and gas wells generates much waste. The wastes are classified as nonhazardous oilfield waste (NOW) and naturally occurring radioactive materials (NORM). NOW originating from drilling and production operations is primarily composed of drill cuttings, sand and spent material such as drilling mud which is no longer suitable for use and must be managed as waste under regulatory authority. Such mud might contain salts, non toxic metals such as sodium and calcium, toxic metals such as barium, chromium, lead, zinc and cadmium, and oil and grease contamination from the introduction of diesel oil (oil based mud), crude oil or a multitude of hydrocarbon based additives. The spent mud, with associated contaminants, comprises a liquid and a solid phase. NOW is also generated in production operations where copious amounts of saline water, along with some solids (sand), may be produced with the desired hydrocarbons. NORM originates primarily from production operations wherein the previously described radioactive scale contaminates not only large pieces of hardware such as well heads and separators but also can contaminate produced "waste" fluids such as salt water and associated solids. It is necessary to dispose of all types of waste, including those previously stored in pits, in a manner which will not contaminate the surface of the earth and not contaminate subterranean aquifers used as sources of drinking water.
Various methods are used to dispose of both NORM and NOW material. Oil and grease toxicity in NOW can be lowered by dilution techniques. Organics can be converted biologically to less toxic forms. Organics can also be removed by extraction processes. These extraction processes can utilize heat and may include methods such as thermal desorption or incineration. Oils can be removed by separation techniques and possibly produce a byproduct of commercial value. Organics can also be bound to solids thereby reducing their leachability and hazard to drinking water supplies. Salts can be diluted and discharged, chemically destroyed or rendered insoluble. Heavy metals can neither be biologically or chemically changed into less toxic species, therefore dilution with non contaminated materials is one method of controlling possible hazardous pollution. Heavy metals can be bound chemically thereby rendering them immobile and nonleachable into the environment. NORM can not be destroyed or chemically altered, therefore dilution with essentially non radioactive material to prescribed levels is an acceptable method. Other possible methods of disposal and/or storage of NORM include near surface burial, deposition with or without encapsulation into the wellbore of plugged and abandoned wells, and injection into geological formations at high pressures which exceed the fracture pressure of the injection formation.
The previous paragraph addresses current practices in the disposal of waste material by type of classification. Another set of disposal criteria has been developed around the physical form of the waste, namely solid or liquid. It should be recalled that spent drilling fluid is in the form of a slurry comprising liquid and solid components. U.S. Pat. No. 4,482,459 (now expired) to Carolyn Shiver, and assigned to the assignee of the current disclosure, teaches a method for continuous processing of a slurry of waste drilling mud fluids and water normally resulting from drilling operations. The process comprises the steps of conducting the drilling mud slurry to a slurry tank for liquid-solid separation by chemical and physical means. The separated solid and liquid components are treated and processed such that they are converted to a state suitable for reuse or release into the environment. There are a number of references which address the separation of liquid and solid components, and the processing of these components to render them harmless to the environment. All of the techniques mentioned above for the disposal of NOW and NORM and the processing of waste slurries are relatively expensive, time consuming, and may involve extensive handling, packaging, transportation and special regulatory permits.
The means of injecting liquid waste back into earth formations by means of a disposal well has been used for many years and remains the predominant method of disposal in the oil and gas industry. An injection well must meet certain criteria. Among these criteria are defined geologic conditions surrounding the injection well, proper casing and cementing of wells penetrating the injection zone, a maximum allowable surface injection pressure (MASIP) and specific procedures for periodic testing and reporting to various regulating agencies. MASIP varies from state to state and even from location to location within a given state dependent upon formation depth, hydrostatic pressure, etc. Being regulatory, MASIP is certainly subject to change in the future. These measures, which are established to prevent possible migration of the waste liquid into underground sources of drinking water (USDW), will be detailed in subsequent sections of this disclosure. Current injection technology requires that the particle size of the solid phase of any slurry first be minimized before injection. This is to prevent clogging or "sanding" of the perforations opposite the injection zone and also to prevent the filling of pore space throats of the injection zone thereby reducing permeability. Processing time and cost must be incurred, and the large particle size solid component of the slurry must still be disposed of in an environmentally acceptable manner. The density of the injected liquid is usually relatively low, varying between 1.00 gm/cc (.about.8.34 lbs/gal) for fresh water to .about.1.1 to 1.2 gm/cc for brines. Often a considerable amount of pump pressure is required to overcome the pressure of the geologic formation and thereby inject the liquid. Adequate pump capacity can comprise an appreciable percentage of the total injection operation cost. In addition, the MASIP is set so as not to damage the tubular strings and the cement sheaths of the injection well and to not damage the injection formation. In some states disposal wells have been drilled into cavities within salt domes or sulfur deposits. In those states cavities are created within salt domes for this purpose, and in the case of sulfur deposits, result from the leach method of production of sulfur. Both of these formations provide impermeable "containers" for liquids but, unfortunately, are not widely distributed geographically and sometimes require that waste be transported a great distance in order to be disposed of in this type of facility.